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WO2006036599A2 - Diodes electroluminescentes presentant une reflectivite elevee et un rendement d'extraction lumineuse eleve - Google Patents

Diodes electroluminescentes presentant une reflectivite elevee et un rendement d'extraction lumineuse eleve Download PDF

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Publication number
WO2006036599A2
WO2006036599A2 PCT/US2005/033210 US2005033210W WO2006036599A2 WO 2006036599 A2 WO2006036599 A2 WO 2006036599A2 US 2005033210 W US2005033210 W US 2005033210W WO 2006036599 A2 WO2006036599 A2 WO 2006036599A2
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WO
WIPO (PCT)
Prior art keywords
light
emitting diode
light emitting
layer
semiconductor structure
Prior art date
Application number
PCT/US2005/033210
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English (en)
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WO2006036599A3 (fr
Inventor
Karl Beeson
Scott Zimmerman
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Goldeneye, Inc.
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Publication date
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Publication of WO2006036599A2 publication Critical patent/WO2006036599A2/fr
Publication of WO2006036599A3 publication Critical patent/WO2006036599A3/fr

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Classifications

    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10H—INORGANIC LIGHT-EMITTING SEMICONDUCTOR DEVICES HAVING POTENTIAL BARRIERS
    • H10H20/00—Individual inorganic light-emitting semiconductor devices having potential barriers, e.g. light-emitting diodes [LED]
    • H10H20/80—Constructional details
    • H10H20/81—Bodies
    • H10H20/813—Bodies having a plurality of light-emitting regions, e.g. multi-junction LEDs or light-emitting devices having photoluminescent regions within the bodies
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10H—INORGANIC LIGHT-EMITTING SEMICONDUCTOR DEVICES HAVING POTENTIAL BARRIERS
    • H10H20/00—Individual inorganic light-emitting semiconductor devices having potential barriers, e.g. light-emitting diodes [LED]
    • H10H20/01—Manufacture or treatment
    • H10H20/011—Manufacture or treatment of bodies, e.g. forming semiconductor layers
    • H10H20/018—Bonding of wafers
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10H—INORGANIC LIGHT-EMITTING SEMICONDUCTOR DEVICES HAVING POTENTIAL BARRIERS
    • H10H20/00—Individual inorganic light-emitting semiconductor devices having potential barriers, e.g. light-emitting diodes [LED]
    • H10H20/80—Constructional details
    • H10H20/81—Bodies
    • H10H20/819—Bodies characterised by their shape, e.g. curved or truncated substrates
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10H—INORGANIC LIGHT-EMITTING SEMICONDUCTOR DEVICES HAVING POTENTIAL BARRIERS
    • H10H20/00—Individual inorganic light-emitting semiconductor devices having potential barriers, e.g. light-emitting diodes [LED]
    • H10H20/80—Constructional details
    • H10H20/85—Packages
    • H10H20/851—Wavelength conversion means
    • H10H20/8511—Wavelength conversion means characterised by their material, e.g. binder
    • H10H20/8512—Wavelength conversion materials
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10H—INORGANIC LIGHT-EMITTING SEMICONDUCTOR DEVICES HAVING POTENTIAL BARRIERS
    • H10H20/00—Individual inorganic light-emitting semiconductor devices having potential barriers, e.g. light-emitting diodes [LED]
    • H10H20/80—Constructional details
    • H10H20/85—Packages
    • H10H20/851—Wavelength conversion means
    • H10H20/8515—Wavelength conversion means not being in contact with the bodies

Definitions

  • LEDs are in contact with air that has a refractive index of 1.00.
  • FIG. 11 is another graph showing the percent reflectivity and the percent extraction efficiency of an array of LEDs as a function of sidewall angle. The output surfaces of the LEDs are embedded in a material having a refractive index of 1.50.
  • FIG. 12 is a graph showing the percent reflectivity and the percent extraction efficiency of an array of LEDs as a function of trench spacing. The output surfaces of the
  • LEDs are embedded in a material that has a refractive index of 1.50.
  • the absorption coefficient alpha is 10 cm "1 and the reflectivity of the reflecting layer is 95%.
  • FIG. 16 is a graph showing the percent reflectivity and the percent extraction efficiency of an array of LEDs as a function of trench spacing. The output surfaces of the
  • An LED of this invention incorporates a multi-layer semiconductor structure that emits light.
  • Inorganic light-emitting diodes can be fabricated from materials containing gallium nitride (GaN), including the materials aluminum gallium nitride (AlGaN) and indium gallium nitride (InGaN).
  • LED materials are aluminum nitride (AlN), aluminum indium gallium phosphide (AlInGaP), gallium arsenide (GaAs), indium gallium arsenide (InGaAs) or indium gallium arsenide phosphide (InGaAsP), for example, but are not limited to such materials.
  • AlN aluminum nitride
  • AlInGaP aluminum indium gallium phosphide
  • GaAs gallium arsenide
  • InGaAs indium gallium arsenide
  • InGaAsP indium gallium arsenide phosphide
  • Especially important LEDs for this invention are GaN-based LEDs that emit light in the ultraviolet, blue, cyan and green region of the optical spectrum and AlInGaP LEDs that emit light in the yellow and red regions of the optical spectrum.
  • GaN-based LEDs that emit light in the ultraviolet, blue, cyan and green region of the optical spectrum
  • FIG. IA is a cross sectional view of an LED die 10, comprising a multi-layer semiconductor structure 12 that is epitaxially grown onto a substrate 20.
  • the multi-layer semiconductor structure contains at least an n-doped GaN layer, a p-doped GaN layer, and an active layer that emits internally generated light.
  • the active layer is illustrated by the dotted line 14.
  • the active layer is typically a GaN-based multi-quantum well structure and is located between the n-doped GaN layer and the p-doped GaN layer.
  • the multi-layer semiconductor structure 12 absorbs light and has an absorption coefficient alpha. In many cases, the absorption coefficient is not uniform across the thickness of the multi-layer semiconductor structure. If the different layers that make up the multi-layer semiconductor structure 12 have different absorption coefficients, the absorption coefficient alpha for the multi-layer semiconductor structure is defined in this specification as the weighted average absorption coefficient. The weighting function is the fractional thickness of each layer in the multi-layer semiconductor structure 12. In GaN-based LEDs, a typical measured absorption coefficient alpha ranges from about 5 cm '1 to about 200 cm " . [0058] Measuring the absorption coefficient alpha for GaN materials can be difficult since GaN materials usually have some scattering in addition to absorption.
  • LED die 30 is inverted and the metal layer 22 is bonded to a sub-mount 26 using a bonding layer 24.
  • the resulting structure is LED 50 illustrated in cross-section in FIG. 1C.
  • the layers of the LED 50 are, in sequence, the sub- mount 26, the bonding layer 24, the metal layer 22, the multi-layer semiconductor structure 12, and the substrate 20.
  • the sub-mount 26 is electrically conducting or contains an electrically conducting layer.
  • the bonding layer 24 is typically an electrically conducting solder.
  • the substrate 20 can be removed from LED 50 to form LED 60 shown in cross section in FIG. ID.
  • the substrate 20 is removed.
  • a laser separation process can be used to remove substrate 20 at the surface 16 of the multi-layer semiconductor structure 12
  • LED 100 illustrated, in plan view in FIG. 2 A.
  • a cross-sectional view in the I-I plane indicated in FIG. IA is illustrated in FIG. 2B.
  • a cross-sectional view in the II-II plane indicated in FIG. IA is shown in FIG. 2C.
  • LED 100 is comprised of a multi-layer semiconductor structure 112 that is in contact with a reflecting layer 122.
  • LED 100 does not include a substrate.
  • a lift-off process is used to remove the substrate onto which the multi-layer semiconductor structure 112 was originally deposited.
  • Reflecting layer 122 is bonded to a sub-mount 126 hy bonding layer 124.
  • LED 100 also includes a second array of light extracting elements, consisting of an array of trenches 140 that is etched through surface 116 and into the multi-layer semiconductor structure 112.
  • the array of trenches 140 is illustrated as substantially perpendicular to the array of trenches 130, but it is not necessary that the two arrays be perpendicular.
  • Adjacent trenches in the array of trenches 140 may be substantially equally, spaced with spacing 146 or may be randomly spaced.
  • the adjacent trenches 140 are substantially equally spaced as indicated in FIG. 2A.
  • the trenches 130 and 140 extend at least part of the way through the multi-layer semiconductor structure 112 and form an array of raised mesas 160.
  • Each raised mesa 160 has a top surface 116, a multi-layer semiconductor layer 112 that includes an active layer 114 and a bottom surface 118.
  • trenches 130 extend only part of the way through the multi-layer semiconductor structure 112.
  • the trenches 130 extend substantially all the way through the multi-layer semiconductor structure 112 but do not extend into the reflecting layer 122.
  • the trenches 130 and trenches 140 extend substantially all the way through the multi-layer semiconductor structure 112 so that light emitted in one mesa will not be transmitted under a trench and through the intervening multi ⁇ layer semiconductor structure 112 to an adjacent mesa.
  • the trenches may extend into the reflecting layer 122, preferably the trenches 130 and 140 do not extend into the reflecting layer 122 and thereby do not cause an undesirable reduction in the reflectivity of LED 100.
  • Trenches may be etched into the multi-layer semiconductor structure 112 using any semiconductor etching technique.
  • Semiconductor etching techniques include reactive ion etching (RIE), laser etching, wet chemical etching and ion milling, but are not limited to these examples.
  • Reflecting layer 122 reflects both internally generated light that is emitted by the active layer 114 and incident light that may enter the LED 100 from external pathways. The incident light may be recycled light that is reflected back to LED 100 after being emitted by LED 100 or the incident light may come from other sources, including other LEDs and phosphors.
  • the reflecting layer 122 is both reflective and electrically conducting. Preferably the reflectivity of reflecting layer 122 is greater than 70%. More preferably, the reflectivity of reflecting layer 122 is greater than 80%. Most preferably, the reflectivity of reflecting layer 122 is greater than 90%.
  • FIGS. 2A and 2C illustrate electrical connections to LED 100.
  • the bottom electrical connections to the mesas 160 are made through bonding pad 150, sub-mount 126, bonding layer 124 and reflecting layer 122.
  • Sub-mount 126 is either fabricated from a material that is electrically conducting or sub-mount 126 contains an electrically conducting layer that conducts electricity from bonding pad 150 to the bonding layer 124.
  • the top electrical connections are made from bonding pads 152 and through electrodes 154 to the top surface 116 of each mesa 160.
  • an insulating layer 156 is first applied to the top surface of LED 100.
  • the insulating layer 156 is patterned to open holes 158 in the insulating layer so that when a conducting metal is deposited onto LED 100, individual electrical contacts are made to the top surface 116 of each mesa 160.
  • a conducting metal layer is deposited and patterned to form individual electrodes 154 as shown in FIG. 2A.
  • the insulating layer 156 may also be removed in areas that will not be covered by electrodes 154.
  • the area of the electrodes 154 should be minimized in order for internally generated light to escape from the uncovered areas of the mesas 160.
  • the electrodes 154 should have high reflectivity in order to efficiently reflect both internally generated light hitting the bottom surfaces of the electrodes 154 and incident light hitting the top surfaces of the electrodes 154.
  • the reflectivity of electrodes 154 is greater than 70%. More preferably, the reflectivity of the electrodes 154 is greater than 80%. Most preferably, the reflectivity of the electrodes 154 is greater than 90%.
  • Preferred electrode metals are aluminum and silver. The more preferred electrode metal is silver.
  • Internally generated light may be emitted from the active layer 114 of LED 100.
  • Two exemplary emitted light rays 170 and 172 are illustrated in FIG. 2E.
  • Light ray 170 is emitted by the active layer 114 and towards surface 116 at an angle less than the critical angle for total internal reflection. Light ray 170 passes through a portion of the multi-layer semiconductor structure 112 until it reaches surface 116. Since light ray 170 strikes the surface 116 at less than the critical angle, light ray 170 will pass through surface 116 and escape from the LED 100 as shown in FIG. 2E.
  • Light ray 172 is emitted from the active layer 114 and towards surface 116 at an angle greater than the critical angle.
  • the number of light extracting elements in this case trenches
  • the spacing between light extracting elements should be decreased.
  • the trench spacing required to achieve high light extraction efficiency depends strongly on the absorption coefficient alpha of the multi-layer semiconductor structure 112 and on the amount of light absorbed by reflections from reflecting layer 122. The minimum amount of light absorption and the maximum light transmission for light rays traveling inside the multi-layer semiconductor structure will occur for light rays that travel in a straight line and that do not reflect from surfaces 116 and 118.
  • the product of alpha times L is 0.1 and alpha is 10 cm “1 , then L is 0.01 cm or 100 microns, T is 90% and 10% of the light is absorbed. [0083] If the absorption coefficient alpha is greater than 10 cm “1 , then the corresponding pathlengh L will be reduced. For example, if the product of alpha times L is 0.4 and alpha is 50 cm “1 , then L is 0.O08 cm or 80 microns, T is 67% and 33% of the light is absorbed. If the product of alpha times L is 0.2 and alpha is 50 cm "1 , then L is 0.004 cm or 40 microns, T is 82% and 18% of the light is absorbed.
  • the spacing 136 and the spacing 146 should be as small as possible, consistent with LED 100 simultaneously achieving acceptable reflectivity to incident light.
  • the light extracting elements are separated by a fractional distance relative to the absorption coefficient alpha. If high reflectivity to incident light can be achieved at the same time, preferably the spacing 136 and the spacing 146 should be less than 0.4 divided by the absorption coefficient alpha. More preferably, the spacing 136 and the spacing 146 should be less than 0.2 divided by the absorption coefficient alpha.
  • LED 100 exhibits high reflectivity to incident light.
  • the reflectivity of LED 100 is different for light rays that strike the flat top surfaces of the mesas 160 compared to light rays that strike the light extraction elements of LED 100. How the reflectivity depends on the point of incidence is illustrated in FIG. 2F by exemplary light rays 180 and 182.
  • LED 100 should preferably exhibit high reflectivity to incident light.
  • the reflectivity of LED 100 is greater than. 70%. More preferably, the reflectivity of LED 100 is greater than 80%. Most preferably, the reflectivity of LED 100 is greater than 90%.
  • the light extraction efficiency is greater than 40%.
  • the sub-area of an LED surface may not liave the same reflectivity.
  • the sub-area of an LED surface covered by electrodes may have a different reflectivity than the sub-area not covered by electrodes. If different sub-areas of an LED surface do not have the same reflectivity, then the reflectivity of the LED is defined in this specification as the weighted average reflectivity for the entire surface of the LED.
  • the weighting function is the fractional portion of the total area of the LED covered by each sub- area.
  • LED 200 illustrated in plan view in FIG. 3 A.
  • a cross-sectional view of LED 200 in the I-I plane is illustrated in FIG. 3B.
  • a cross- sectional view in the H-II plane is shown in FIG. 3C.
  • LED 300 is comprised of a multi-layer semiconductor structure 212 that is in contact with a reflecting layer 222.
  • Preferably LED 200 does not include a substrate.
  • Reflecting layer 222 is bonded to a sub-mount 226 by bonding layer 224.
  • An array of light extracting elements, consisting of an array of trenches 230, is etched through surface 216 and into the multi-layer semiconductor structure 212. Adjacent trenches may be substantially equally spaced with spacing 236 or may be randomly spaced.
  • the adjacent trenches 230 are substantially equally spaced as shown in FIGS. 3A-3C.
  • the sidewalls 232 and 234 of the trenches 230 are illustrated as flat surfaces, but the sidewalls may be either flat or curved. Sidewalls 232 and 234 are tilted at angle 238, measured from a direction perpendicular to surface 216. In FIG. 3B, the sidewall slope is defined as a negative slope and the angle 238 is defined as a negative angle.
  • LED 200 also includes a second array of light extracting elements, consisting of an array of trenches 240 that is etched through surface 216 and into the multi-layer semiconductor structure 212.
  • the array of trenches 240 is illustrated as substantially perpendicular to the array of trenches 230, but it is not necessary that the two arrays be perpendicular. Adjacent trenches in the array of trenches 240 may be substantially equally spaced with spacing 246 or may be randomly spaced. Preferably, the adjacent trenches 240 are substantially equally spaced as indicated in FIG. 3 A.
  • Reflecting layer 222 reflects both internally generated light that is emitted by the active layer 214 and incident light that may enter the LED 100 from external pathways.
  • the reflecting layer 222 is both reflective and electrically conducting.
  • the reflectivity of reflecting layer 222 is greater than 70%. More preferably, the reflectivity of reflecting layer 222 is greater than 80%. Most preferably, the reflectivity of reflecting layer 222 is greater than 90%.
  • FIGS. 3A and 3C illustrate electrical connections to LED 200.
  • the bottom electrical connections to the mesas 260 are made through bonding pad 250, sub-mount 226, bonding layer 224, and reflecting layer 222.
  • the top electrical connections are made from bonding pads 252 and through electrodes 254 to the top surface 216 of each mesa 260.
  • an insulating layer 256 is first applied to the top surface of LED 200.
  • the insulating layer 256 is patterned to open holes 258 in the insulating layer so that when a conducting metal is deposited onto LED 200, individual electrical contacts are made to the top surface 216 of each mesa 260.
  • a conducting metal layer is deposited and patterned to form individual electrodes 254 as shown in FIG. 3 A.
  • the insulating layer 256 may also be removed in areas that will not be covered by electrodes 254.
  • the area of the electrodes 254 should be minimized in order for internally generated light to escape from the uncovered areas of the mesas 260.
  • the electrodes 254 should have high reflectivity in order to efficiently reflect both internally generated light hitting the bottom surfaces of the electrodes 254 and incident light hitting the top surfaces of the electrodes 254.
  • the reflectivity of electrodes 254 is greater than 70%. More preferably, the reflectivity of the electrodes 254 is greater than 80%. Most preferably, the reflectivity of the electrodes 254 is greater than 90%.
  • Preferred electrode metals are aluminum and silver. The more preferred electrode metal is silver.
  • the material for the electrodes 254 can be a transparent conductor. If the material for the electrodes 254 is a transparent conductor, the light transmission of the transparent conductor is preferably greater than 90%.
  • Light such as internally generated light ray 272 and incident light ray 282 may be trapped inside the multi-layer semiconductor structure for a sufficient distance so that a significant portion of the light is absorbed.
  • LED 100 it is desirable that internally generated light rays that are trapped in LED 200 by total internal reflection travel only a short distance before exiting a light extracting means. This requires that trenches 230 and 240 should be closely spaced.
  • closely spaced trenches in LED 200 will also result in more incident light rays following paths such as light ray 282, resulting in lower reflectivity for LED 200. Designing LED 200 so that it has both acceptable high reflectivity and acceptable high light extraction efficiency will again require some compromise between the competing requirements.
  • LED 200 should preferably exhibit high reflectivity to incident light.
  • the reflectivity of LED 200 is greater than 70%. More preferably, the reflectivity of LED 200 is greater than 80%. In addition, preferably the extraction efficiency is greater than 40%.
  • Another embodiment of this invention is LED 300, illustrated in plan view in FIG. 4A. A cross-sectional view of LED 300 in the I-I plane is illustrated in FIG. 4B. A cross- sectional view in the IMI plane is shown in FIG. 4C.
  • LED 300 is similar to LED 100 and LED 200 except that the light extracting elements in LED 300 are an array of holes 330.
  • Holes 330 are etched at least part way through multi-layer semiconductor structure 312 by the etching methods previously listed. Preferably, the holes are etched substantially all the way through multi-layer semiconductor structure 312.
  • the holes 330 are illustrated with a round cross-section, but the holes may have any cross-sectional shape including a circle, an ellipse, an arbitrary curved shape, a square, a rectangle or a polygon.
  • Adjacent holes may be substantially equally spaced with spacing 336 or may be randomly spaced.
  • the adjacent holes 330 are substantially equally spaced as shown in FIGS. 4A-4C.
  • the sidewalls 332 of the holes 330 are illustrated as having a linear taper, but the sidewalls may also be tapered in a non-linear manner.
  • LED 300 also includes a reflecting layer 322 that is bonded to a sub-mount 326 by bonding layer 324.
  • Reflecting layer 322 reflects both internally generated light that is emitted by the active layer 314 and incident light that may enter the LED 300 from external pathways.
  • the reflectivity of reflecting layer 322 is greater than 70%. More preferably, the reflectivity of reflecting layer 322 is greater than 80%. Most preferably, the reflectivity of reflecting layer 322 is greater than 90%.
  • FIGS. 4A and 4C illustrate electrical connections to LED 300.
  • the bottom electrical connections are made through bonding pad 350, sub-mount 326, bonding layer 324 and reflecting layer 322.
  • the top electrical connections are made from bonding pads 352 and through electrodes 354 to the top surface 316 of LED 300.
  • an insulating layer 356 is fabricated between electrode 352 and sub-mount 326. Electrodes 354 are formed by depositing a conducting metal layer and patterning the layer to form individual electrodes 354 as shown in FIG. 4A.
  • the area of the electrodes 354 should be minimized in order for internally generated light to escape from the uncovered areas of LED 300.
  • the electrodes 354 should have high reflectivity in order to efficiently reflect both internally generated light hitting the bottom surfaces of the electrodes and incident light hitting the top surfaces of the electrodes.
  • the reflectivity of electrodes 354 is greater than 70%. More preferably, the reflectivity of the electrodes 354 is greater than 80%. Most preferably, the reflectivity of the electrodes 354 is greater than 90%.
  • Preferred electrode metals are aluminum and silver. The more preferred electrode metal is silver.
  • the material for the electrodes 354 may also be a transparent conductor.
  • Light extraction element spacing 336 on LED 300 affects both the light extraction efficiency of internally generated light and the reflectivity of incident light.
  • Three exemplary light rays 372, 380 and 382 in FIG. 4B illustrate these effects.
  • Light ray 372 is an internally generated light ray and light rays 380 and 382 are reflected incident light rays.
  • Light ray 372 is emitted from the active layer 314 and directed towards surface 316 at an angle greater than the critical angle. Light ray 372 passes through a portion of the multi-layer semiconductor structure 312 until it reaches surface 316. Since light ray 372 strikes the surface 316 at an angle greater than the critical angle, light ray 372 is reflected by surface 316. Light ray 372 undergoes an additional reflection from reflecting layer 322 before exiting tapered surface 332 of hole 330.
  • Light rays 380 and 382 are incident light rays that are reflected by LED 300. Light ray 380 is incident on surface 316 in an area of LED 300 that contains no light extracting elements.
  • Light such as internally generated light ray 372 and incident light ray 382 may be trapped inside the multi-layer semiconductor structure for a sufficient distance so that a significant portion of the light is absorbed.
  • closely spaced light extracting elements in LED 300 will also result in more incident light rays following paths such as light ray 382, resulting in lower reflectivity for LED 300.
  • the requirements and preferred characteristics of spacing 336 for LED 300 are identical to the requirements and preferred characteristics of spacing 136 and spacing 146 for LED 100.
  • the reflectivity of LED 300 is greater than 70%. More preferably, the reflectivity of LED 300 is greater than 80%. In addition, preferably the extraction efficiency is greater than 40%.
  • LED 400 illustrated in plan view in FIG. 5 A.
  • a cross-sectional view of LED 400 in the I-I plane is illustrated in FIG. 5B.
  • a cross- sectional view in the II-II plane is shown in FIG. 5 C.
  • LED 500 is similar to the previous embodiments except that the light extracting elements in LED 400 are a first array of etched strips 430 and a second array of etched strips 440.
  • Etched strips 430 and 440 are roughened areas etched in the surface 416 by the etching methods previously listed.
  • the etched strips 43 O and 440 are etched in the surface 416 of multi-layer semiconductor structure 412 by a wet etch process utilizing potassium hydroxide.
  • the etched strips 430 may have substantially equal spacing 436 or may be randomly spaced.
  • the adjacent etched strips 430 have substantially equal spacing as shown in FIGS. 5A-5C.
  • the etched strips 440 have similar characteristics.
  • LED 400 also includes a reflecting layer 422 that is bonded to a sub-mount 426 by bonding layer 424.
  • Reflecting layer 422 reflects both internally generated light that is emitted by the active layer 414 and incident light that may enter the LED 400 from external pathways.
  • the reflectivity of reflecting layer 422 is greater than 70%. More preferably, the reflectivity of reflecting layer 422 is greater than 80%. Most preferably, the reflectivity of reflecting layer 422 is greater than 90%.
  • FIGS. 5A and 5C illustrate electrical connections to LED 400.
  • the bottom electrical connections are made through bonding pad 450, sub-mount 426, bonding layer 424 and reflecting layer 422.
  • the top electrical connections are made from bonding pads 452 and through electrodes 454 to the top surface 416 of LED 400.
  • an insulating layer 456 is fabricated between electrode 454 and the top surface 416 of LED 400. Electrodes 454 are formed by depositing a conducting metal layer and patterning the layer to form individual electrodes 454 as shown in FIG. 5A. [0122] The area of the electrodes 454 should be minimized in order for internally generated light to escape from the uncovered areas of LED 400.
  • the reflectivity of electrodes 454 is greater than 7O%. More preferably, the reflectivity of the electrodes 454 is greater than 80%. Most preferably, the reflectivity of the electrodes 454 is greater than 90%. As in the previous embodiments, the material for the electrodes 454 may also be a transparent conductor.
  • Light extraction element spacing 436 and spacing 446 on LED 400 affect both the light extraction efficiency of internally generated light and the reflectivity of incident light.
  • Three exemplary light rays 472, 480 and 482 in FIG. 5B illustrate these effects.
  • Light ray 472 is an internally generated light ray and light rays 480 and 482 are reflected incident light rays.
  • Light ray 472 is emitted from the active layer 414 and directed towards surface 416 at an angle greater than the critical angle. Light ray 472 is temporarily trapped in the multi-layer semiconductor structure 412 by total internal reflection. Light ray 472 is reflected twice from surface 416 and twice from reflecting layer 422 before exiting LED 400 at an adjacent etched strip 430. Alternatively, light ray 472 may pass under the adjacent etched strip 430 and travel to another etched strip 430 before exiting LED 400. [0125] Since light ray 472 or similar rays may be temporarily trapped by total internal reflection within the multi-layer semiconductor structure 412, a significant portion of light ray 472 may be absorbed by the multi-layer semiconductor structure 412 and by the reflecting layer 422. As in the previous embodiments, in order to minimize the absorption losses experienced by trapped light and to maximize light emission from LED 400, the number of light extracting elements (in this case etched strips) should be increased and the spacing between light extracting elements should be decreased.
  • Light rays 480 and 482 are incident light rays that are reflected by LED 400.
  • Light ray 480 is incident on surface 416 in an area of LED 400 that contains no light extracting elements.
  • Light ray 480 passes through the multi-layer semiconductor structure 412 only twice and is reflected by reflecting layer 422 only once, so that absorption losses will be relatively small and the percent reflected will be relatively high.
  • Light ray 482 is incident on etched strip 430, is transmitted into the multi-layer semiconductor structure 412, and is reflected by reflecting layer 422.
  • Light ray 482 passes through the multi-layer semiconductor structure 412 to surface 416 and is reflected by total internal reflection if the angle relative to surface 416 is greater than the critical angle.
  • Light ray 482 is trapped inside the multi-layer semiconductor structure until it is either absorbed or until it reaches the surface of an adjacent etched strip. If ray 482 reaches an adjacent etched strip, it may exit LED 400 through the etched strip.
  • Light such as internally generated light ray 472 and incident light ray 482 may be trapped inside the multi-layer semiconductor structure for a sufficient distance so that a significant portion of the light is absorbed.
  • internally generated light rays that are trapped in LED 400 by total internal reflection travel only a short distance before exiting a light extracting means. This requires that etched strips 430 and 440 should be closely spaced.
  • closely spaced light extracting elements in LED 400 will also result in more incident light rays following paths such as light ray 482, resulting in lower reflectivity for LED 400.
  • spacing 436 and spacing 446 for LED 400 are identical to the requirements and preferred characteristics of spacing 136 and spacing 146 for LED 100. Designing LED 400 so that it has both acceptable high reflectivity and acceptable high light extraction efficiency is possible but will again require some compromise between the competing requirements.
  • the reflectivity of LED 400 is greater than 70%. More preferably, the reflectivity of LED 400 is greater than 80%. In addition, preferably the extraction efficiency Is greater than 40%.
  • Another embodiment of this invention is LED 500.
  • LED 500 is illustrated in plan view in FIG. 6A and in cross-section in FIGS. 6B and 6C. LED 500 is similar to LED 100 except that the light extracting elements in LED 500 are raised ridges instead of trenches.
  • LED 500 is comprised of a multi-layer semiconductor structure 512 that is in contact with a reflecting layer 522.
  • LED 500 does not include a substrate.
  • Reflecting layer 522 is bonded to a sub-mount 526 by bonding layer 524.
  • An array of light extracting elements consisting of an array of ridges 530, is fabricated on surface 516 of the multi-layer semiconductor structure 512 by standard deposition and lithographic techniques.
  • the ridges 530 may be fabricated from a different material than trie multi-layer semiconductor structure. Adjacent ridges may be substantially equally spaced with spacing 536 or may be randomly spaced.
  • the sidewalls 532 and 534 of the ridges 530 are illustrated as flat surfaces, but the sidewalls maybe either flat or curved.
  • LED 500 also includes a second array of light extracting elements, consisting of an array of ridges 540 that is fabricated on surface 516 of the multi-layer semiconductor structure 512.
  • the arra.y of ridges 540 is illustrated as substantially perpendicular to the array of trenches 53 O, but it is not necessary that the two arrays be perpendicular. Ridges 530 and 540 improve the light extraction efficiency of LED
  • the spacing 536 of ridges 530 and the spacing 546 of ridges 540 on LED 500 affect both the light extraction efficiency of internally generated light and the reflectivity of incident light.
  • Three exemplary light rays 572, 580 and 582 in FIG. 6B illustrate these effects.
  • Light ray 572 is an internally generated light ray and light rays 580 and 582 are reflected incident light rays.
  • Light ray 572 is emitted from the active layer 514 and directed towards surface
  • Light ray 572 is temporarily trapped in the multi-layer semiconductor structure 512 by total internal reflection. Light ray 572 is reflected twice from surface 516 and twice from reflecting layer 522 before exiting LED 500 through surface 534b of an adjacent ridge 530. Alternatively, light ray 572 may pass under the adjacent ridge 530 and travel to another ridge 530 before exiting LED 50O.
  • Light rays 580 and 582 are incident light rays that are reflected by LED 500.
  • Light ray 580 is incident on surface 516 in an area of LED 500 that contains no light ridges.
  • Light ray 580 passes through the multi-layer semiconductor structure 512 only twice and is reflected by reflecting layer 522 only once, so that absorption losses will TDe relatively small and the percent reflected will be relatively high.
  • Light ray 582 is incident on surface 534a of ridge 530, is transmitted into the multi-layer semiconductor structure 512, and is reflected by reflecting layer 522.
  • Light ray 582 is incident on surface 534a of ridge 530, is transmitted into the multi-layer semiconductor structure 512, and is reflected by reflecting layer 522.
  • Light such as internally generated light ray 572 and incident light ray 582 may be trapped inside the multi-layer semiconductor structure 512 for a sufficient distance so that a significant portion of the light is absorbed.
  • closely spaced light extracting elements in LED 500 will also result in more; incident light rays following paths such as light ray 582, resulting in lower reflectivity for LED 500.
  • Light emitting diodes that exhibit both high reflectivity to incident light and high extraction efficiency to internally generated light may be utilized in illumination systems that reflect and recycle a portion of the light generated by the light emitting diodes back to the light emitting diodes.
  • Light recycling illumination systems have been disclosed in U.S. Patent Application Ser. No. 10/445,136, U.S. Patent Application Ser. No. 10/814,043 and U.S. Patent Application Ser. No. 10/814,044.
  • Examples of exemplary light recycling illumination systems that include a light recycling means are illustrated in FIGS. 7 A, 7B, SA, 8B and 9.
  • all the illumination system figures illustrate LEDs having the trench configuration of LED 100.
  • LEDs having the configurations of LED 200, LED 300, LED 400 and LED 500 may also be utilized in the illustrated illumination systems.
  • FIG. 7 A A cross-sectional view of another embodiment of this invention is illustrated in FIG. 7 A.
  • Illumination system 600 in FIG. 7 A is comprised of LED 100 (illustrated in FIGS. 2A-2F) and a reflecting polarizer 610.
  • Reflecting polarizer 610 transmits a first polarization state of light emitted by active layer 114 of LED 100 and reflects a second polarization state of light emitted by the active layer.
  • the polarization states may be linear polarization states or circular polarization states.
  • Exemplary reflecting polarizers are polarizers made by NanoOpto Corporation and Moxtek Incorporated that utilize subwavelength optical elements or wire-grid optical elements.
  • Light rays 612 and 614 illustrate the operation of illumination system 600.
  • Light ray 612 of a first polarization state is emitted by active layer 114 of LED 100 and directed towards surface 116 at an angle less than the critical angle.
  • Light ray 612 is transmitted through surface 116 and is transmitted by reflecting polarizer 610.
  • Light ray 614 of a second polarization state is emitted by active layer 114 of LED 100 towards surface 116 at an angle less than the critical angle.
  • Light ray 614 of a second polarization state is transmitted by surface 116 and is directed towards reflecting polarizer 610.
  • Reflecting polarizer 610 reflects light ray 614 of a second polarization state back to LED 100.
  • a fraction of light ray 614 of a second polarization state will reflect from the reflecting layer 122 of LED 100 and increase the effective brightness of LED 100.
  • Light of a second polarization state that is reflected from LED 100 may be partly converted to light of a first polarization state. Such converted light can then pass through reflecting polarizer 610 as light of a first polarization state and thereby increase the overall efficiency of illumination system 600.
  • FIG. 7B is a cross-sectional view of another embodiment of this invention, illumination system 650.
  • Illumination system 650 is similar to illumination system 600 but further comprises a light collimating means.
  • the light collimating means may be, for example, a tapered waveguide, a compound parabolic reflector, a lens or a combination of two or more such elements.
  • the light collimating means is a tapered waveguide 660, which has an input surface 662 and an output surface 664. In order for the tapered waveguide 660 to partially collimate light, the area of the output surface 664 must be larger than the area of the input surface 662.
  • Light rays 672 and 674 illustrate the operation of illumination system 650.
  • Light ray 672 of a first polarization state is emitted by active layer 114 of LED 100 towards surface 116 at an angle less than the critical angle.
  • Light ray 672 of a first polarization state is transmitted through surface 116 and enters tapered waveguide 660 through input surface 662.
  • Light ray 672 of a first polarization state is partially collimated by reflecting from a side surface 666 of the tapered waveguide 660.
  • Light ray 672 of a first polarization state exits tapered waveguide 660 through output surface 664 as partially collimated light and is transmitted by reflecting polarizer 610.
  • Light ray 674 of a second polarization state is emitted by active layer 114 of LED 100 towards surface 116 at an angle less than the critical angle.
  • Light ray 674 of a second polarization state is transmitted through surface 116 and enters tapered waveguide 660 through input surface 662.
  • Light ray 674 of a second polarization state is partially collimated by reflecting from a side surface 666 of the tapered waveguide 660.
  • Light ray 674 of a second polarization state exits tapered waveguide 660 through output surface 664 as partially collimated light and is directed to reflecting polarizer 610.
  • Reflecting polarizer reflects light ray 674 of a second polarization state back to LED 100.
  • a fraction of light ray 674 of a second polarization state will reflect from the reflecting layer 122 of LED 100 and increase the effective brightness of LED 100.
  • Light of a second polarization state that is reflected from LED 100 may be partly converted to light of a first polarization state and can then pass through reflecting polarizer 610. Such reflected and converted light can increase the overall output efficiency of illumination system 650.
  • illumination system 700 incorporates a light recycling envelope and is illustrated in plan view in FIG. 8 A.
  • FIG. 8B shows a cross- sectional view along the I-I plane indicated in FIG. 8 A.
  • Illumination system 700 incorporates five LEDs that are identical to LED 100 shown in FIGS. 2A-2F.
  • LED 100 is chosen for illustrative purposes.
  • LEDs 200, 300, 400 and 500 may also be used in this embodiment.
  • the number of LEDs utilized may be more or less than five. For example, one may place four LEDs on each of the five sides of the light recycling envelope for a total of 20 LEDs.
  • the five LEDs are mounted on the inside surfaces of a light recycling envelope 710.
  • the light recycling envelope 710 is closed on each of the five sides that contains an LED and is open on the sixth side.
  • the open sixth side forms output aperture 720 as illustrated in the cross-sectional view in FIG. 8B.
  • LED 100a and LED 100b are mounted on opposing inside surfaces 712 of light recycling envelope 710.
  • LEDs lOOd and 11Oe are mounted on opposing inside surfaces 712 and are perpendicular to LEDs 100a and 100b.
  • LED 100c is mounted on the fifth inside surface 712. Electrodes for LEDs 100a, 100b, lOOd and lOOe are not shown in order to simplify the diagram. Wire bonds to connect the LEDs to a power supply are also not shown.
  • the LEDs and all the exposed surfaces on the inside of light recycling envelope 710 reflect light.
  • Exposed surfaces include surfaces 712, 190a, 190b, 190c, 19Od, 19Oe, 192a, 192b, 192c, 192d and 192e.
  • the LEDs and all exposed surfaces on the inside of light recycling envelope 710 have reflectivity greater than 70%. More preferably, the LEDs and all exposed surfaces on the inside of light recycling envelope 710 have reflectivity greater than 80%.
  • Light ray 730 is emitted by active layer 114a of LED 100a. Light ray 730 exits surface 116a of LED 100a and exits illumination system 700 through output aperture 720.
  • Light ray 740 is emitted by active layer 114a of LED 100a. Light ray 740 passes through the interior of light recycling envelope 710 to LED 100c. Light ray 740 passes through surface 116c and is reflected and recycled by reflecting layer 122c. The reflection of light ray 740 by reflecting layer 122c of LED 100c increases the effective brightness of LED 100c.
  • light can reflect and recycle from the surfaces of the other LEDs and increase the effective brightness of the other LEDs.
  • Light ray 740 exits LED 100c through surface 116c and exits the light recycling envelope 710 through output aperture 720.
  • the total area of the output aperture 720 is less than the total emitting area of the five LEDs, light recycling can increase the output brightness of illumination system 700 to a value that is greater than the output brightness of one of the LEDs in the absence of recycling. Illumination systems with increased output brightness are useful for applications such as projection displays.
  • Illumination system 800 illustrated in cross section in FIG. 9, is another embodiment of this invention.
  • the light recycling means is a wavelength conversion layer 810 that converts a portion of the light internally generated in LED 100 into light having a different wavelength.
  • the elements of LED 100 have been described previously.
  • the wavelength conversion layer 810 is comprised of a phosphor material, a quantum dot material or a plurality of such materials.
  • the wavelength conversion layer may further comprise a transparent host material into which the phosphor material or the quantum dot material is dispersed.
  • Phosphor materials are typically optical inorganic materials doped with ions of lanthanide (rare earth) elements or, alternatively, ions such as chromium, titanium, vanadium, cobalt or neodymium.
  • the lanthanide elements are lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium.
  • Optical inorganic materials include, but are not limited to, sapphire (Al 2 O 3 ), gallium arsenide (GaAs), beryllium aluminum oxide (BeAl 2 O 4 ), magnesium fluoride (MgF 2 ), indium phosphide (InP), gallium phosphide (GaP), yttrium aluminum garnet (YAG or Y 3 Al 5 O 12 ), terbium-containing garnet, yttrium-aluminum- lanthanide oxide compounds, yttrium-aluminum-lanthanide-gallium oxide compounds, yttrium oxide (Y 2 O 3 ), calcium or strontium or barium halophosphates (Ca,Sr,Ba) 5 (PO 4 ) 3 (Cl,F), the compound CeMgAl 11 O 1 ⁇ lanthanum phosphate (LaPO 4 ), lanthanide pentaborate materials ((lanthanide)(Mg,Zn)B
  • An exemplary red emitting phosphor is Y 2 O 3 :Eu 3+ .
  • An exemplary yellow emitting phosphor is YAGiCe 3+ .
  • Exemplary green emitting phosphors include CeMgAl ⁇ 0 19 :Tb 3+ , ((lanthanide)PO 4 : Ce 3+ Jb 3+ ) and GdMgB 5 Oi 0 :Ce 3+ ,Tb 3+ .
  • Exemplary blue emitting phosphors are BaMgAl 10 O 17 :Eu 2+ and (Sr,Ba,Ca) 5 (PO 4 ) 3 Cl:Eu 2+ .
  • exemplary optical inorganic materials include yttrium aluminum garnet (YAG or Y 3 Al 5 On), terbium-containing garnet, yttrium oxide (Y 2 O 3 ), YVO 4 , SrGa 2 S 4 , (Sr,Mg,Ca,Ba)(Ga,Al,In) 2 S 4 , SrS, and nitridosilicate.
  • Example 5 is similar to Example 3 except that the absorption coefficient of the 4 micron thick GaN multi-layer semiconductor structure was reduced by a factor of five to a value of 10 cm "1 .
  • the reflectivity R(metal) of the reflecting layex remained at 95%.
  • the output side of the LED was in contact with air having a refractive index n of 1.0.
  • the results are graphed in FIG. 14. Lowering the absorption coefficient improved both the LED extraction efficiency 1410 and the LED reflectivity 1420 compared with FIG. 12 in Example 3. Reflectivity values were greater than 80% when the trench spacing L was greater than about 60 microns.

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Abstract

Cette invention concerne une diode électroluminescente présentant une réflectivité élevée à la lumière incidente et un rendement d'extraction lumineuse élevé pour une lumière émise de l'intérieur. La diode électroluminescente décrite dans cette invention comprend une couche réfléchissante qui reflète à la fois la lumière incidente et la lumière émise de l'intérieur. Une structure semi-conductrice multicouche est déposée sur la couche réfléchissante. Cette structure semi-conductrice multicouche comprend une couche active qui émet la lumière émise de l'intérieur. Une matrice d'éléments extracteurs de lumière s'étend au moins en partie à travers la structure semi-conductrice multicouche et elle améliore le rendement d'extraction de la lumière émise de l'intérieur. Les éléments extracteurs de lumière peuvent être une matrice de tranchées, une matrice de trous, une matrice de crêtes ou une matrice de bandes gravées. La diode électroluminescente décrite dans cette invention améliore le rendement de systèmes d'éclairage à recyclage de lumière.
PCT/US2005/033210 2004-09-28 2005-09-16 Diodes electroluminescentes presentant une reflectivite elevee et un rendement d'extraction lumineuse eleve WO2006036599A2 (fr)

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